Researchers　and　engineers　are　constantly　developing　novel　techniques　to　accurately　measure　the　electromagnetic　properties　of　electrical　insulators.　These　properties　provide　useful　information　about　the　insulators＇　performance　and　aging　degree,　which　may　help　users　to　take　necessary　actions　to　prevent　potential　risks　to　the　safety　and　stability　of　electric　power　systems.　In　this　study,　terahertz　(THz)　time-domain　spectroscopy　(THz-TDS)　is　used　to　analyze　the　spectral　absorption　and　dielectric　dispersion　of　five　electrical　insulation　materials　viz.　epoxy　resin　(PER),　E-glass　fiber-reinforced　polymer-matrix　composite　(GFRP),　cross-linked　polyethylene　(XLPE),　electrical　porcelain　(E-PRL),　and　high　temperature　vulcanized　silicone　rubber　(HTVSR),　in　the　THz　frequency　range　(0.1　THz-1.8　THz).　The　refractive　index,　absorption　coefficient,　and　complex　relative　permittivity　of　each　sample　are　experimentally　measured　using　the　THz-TDS　system　and　theoretically　calculated　using　the　original　Debye　model　(i.e.,　based　on　the　relaxation　process　of　the　individual　samples’　dipoles).　Experimental　and　theoretical　results　of　the　refractive　indices　and　dielectric　constants　of　the　individual　samples　are　highly　correlated　with　0.22%　and　0.17%　aggregate　maximum　errors,　respectively.　The　values　of　dielectric　constants　and　imaginary　permittivities　are　found　to　be　the　highest　and　the　lowest　for　E-PRL　and　XLPE,　respectively.　In　terms　of　dielectric　performance,　XLPE　exhibits　the　highest　and　most　stable　dielectric　performance,　followed　by　the　E-PRL,　whereas　PER,　HTVSR,　and　GFRP　exhibit　fluctuating　and　slightly　less　stable　dielectric　performance.　This　study　shows　the　great　potential　of　THz　nondestructive　testing　(NDT)　and　structural　health　monitoring　(SHM)　for　the　defect　detection　and　service　life　prediction　of　electrical　insulators.　©　2021　Elsevier　B.V.